What is PSS memory?

Measuring and understanding how a process is using the primary memory (aka RAM) is complex because Operating System have been optimising primary memory use to save space or optimise run time (memory map, virtual memory, kernel same page merging, etc.). Therefore most tasks managers are reporting several metrics to inform about memory usage, and even their own help page are sometimes reporting a wrong definition for a metric. So I am always learning new things about what actually all those metrics are measuring. Today’s is PSS:

Linux’s PSS (Proportional Set Size) metric. This is the amount of RAM actually mapped into the process, but weighted by the amount it is shared across processes. So if there is a 4K page of RAM mapped in to two processes, its PSS amount for each process would be 2K. The nice thing about using PSS is that you can add up this value across all processes to determine the actual total RAM use. – Android Developer Blog, Understanding How Your App Uses RAM